Coalescing demister

ABSTRACT

The specification discloses a novel Coalescing Demister having, depending on the direction of air flow, an inner or outer prefilter-precoalescer subassembly consisting of one or more layers of glass fiber cloth sandwiched between two screen-type retainers, and an inner or outer coalescing subassembly consisting of an inner and outer perforated retainer having a formed glass fiber coalescing media formed there between, together with a layer of synthetic cloth to act as an antichanneling layer, if desired. The prefilter-precoalescer subassembly, and the coalescing subassembly with drain and anti-migration layers are potted between two end caps with suitable openings therein for the normal oil scavenging tubes. An air gap between the two subassemblies is physically maintained by the end caps, and an air flow barrier is provided around the lower periphery of the inner or outer perforated retainer, depending on the direction of air flow, to provide for accumulation of the coalesced liquid and its flow by gravity back through the prefilter-precoalescer subassembly against the air flow.

The present application is a continuation in part of our earlierco-pending application, Ser. No. 593,831, filed July 7, 1975, for"Composite Coalescing Filter Tube and Method of Manufacture Thereof",now U.S. Pat. No. 4,078,965.

The present application relates to demisters of the type frequenctlyused in air compressors, and in any other application, where it isdesired to remove quantities of oil or other liquids which have becomeentrained in an air or gas stream, and more particularly to a coalescingtype demister of the type having a prefilter-precoalescer stage and aregular coalescing stage.

It should be understood that a demister is something rather old in thefilter art, with the need for the same being at least as old as thefirst practical air or gas compressor. The function of the demister isto remove the oil used to lubricate the compressor itself from thecompressed air or gas stream leaving the compressor, and to return it tothe oil reservoir thereof. Without the use of the demister, thelubricating oil would absent itself from the oil reservoir, resulting ina costly breakdown of the compressor. Further, large amounts of oil inthe compressed air or gas stream are very damaging to the operation ofmany types of air operated machinery, and thus, elimination of entrainedoil has been essential for their proper operation.

It is to be noted that the terms "air stream", "gas stream" and "air orgas stream" are used interchangeably throughout the specification, andit is to be further understood that whenever any one of these terms areused, such term should be read as referring to an "oil or gas stream".

Similarly, when the term "compressor" is used, this term should beinterpreted to refer to an "air or gas compressor".

In the earlier days of the compressor, wherein low pressures wereinvolved, it was very satisfactory to use a crude demister of the typewhich just involved passing the air or gas through a fibrous filtermaterial, such as lamb's wool, to remove the largest droplets of oilfrom the air stream, which were in the neighborhood of 1-50 microns insize. Although the lamb's wool or other material would rather quicklybecome saturated with oil, such material was inexpensive, and could beeasily replaced if required.

However, the development of more and more sophisticated air operatedmachinery necessitated the development of compressors operating at everhigher pressures and temperatures which rendered such crude demisterstotally unuseable and led to research, which is continuing to be presentday, directed to making a satisfactory demister which not only removesdroplets of oil from the air stream, but which will also remove what isknown in the art as "oil mist" or "oil aerosols" from the air or gasstream, which involves oil droplets of 1/10 to 1 micron in diameter.

All of the demisters currently on the market have attempted to providesuch a demister by introducing the air stream first to a very tightcoalescing media, with the intention that very large drops of oil willform as the air stream passes through such coalescing media, suchdroplets of oil hopefully containing a major portion of the oilentrained in the air stream, with the air then being passed through arelatively thin drain layer which is provided beyond the coalescinglayer, which would again, hopefully, remove additional oil. While thisproduced a demister which was satisfactory for many purposes, until thepresent invention the best possible demister, when used in conjunctionwith the most modern type of compressor, such as the rotary screw typecompressor, still left an unsatisfactory amount of oil in the air.

Further, attempts to improve performance of such demisters took the formof providing an air gap between the very tight coalescing layer of thefilter and the drain layer in the hope that additional oil would settleout of the air stream during its travel through the air gap. However, itis generally conceded that this hope was not realized, and oil was stillleft in the air stream in unsatisfactory amounts.

With the advent of such rotary air compressors operating at very highpressure and temperatures, the available demisters became even lessdesirable, and in some cases it was even found that some component partsdeteriorated under the high temperatures and pressures encountered, andthis added even more urgency to the search for a demister which wouldoperate satisfactorily under these conditions for a prolonged period oftime. We, being already experienced in the art of manufacturingcoalescing filter tubes, believed that much the same principles whichled to the successful operation of our filter tube could be applied toproducing such a demister. In out search for a successful demister wefirst used the theory of operation of the demister devices currentlyavailable which all operate, as previously mentioned, on the theory thatyou must immediately pass the air stream through a very tight coalescinglayer.

In our studies we noted that this method of operation led to the veryearly complete saturation of these tight coalescing layers, resulting ina decrease of efficiency and early failure.

The demisters which were provided with the air gaps previously discussedhad the same problems, and were unsatisfactory for this reason. Aftermuch experimentation, we arrived at the conclusion that the theory onwhich all present demisters operate was basically wrong, and that aprefilter-precoalescer function should be performed, which would removealmost as much oil from the air stream as the much tighter coalescinglayer, and would provide for the concentration of the entrained oil inthe air stream reaching the coalescing layer to be much less, leavingthe coalescing layer free to perform its function much more efficiently,thus resulting in a much lower concentration of oil remaining in the airstream leaving the demister.

In our solution to the problem of providing a satisfactory demister, wethus provided an outer prefilter-precoalescer subassembly through whichthe air stream must pass before entering a concentrated inner coalescingsubassembly. With the provision of suitable end caps and scavenging, ithas been found that the oil passing through the prefilter-precoalescersubassembly will drain from the prefilter layer into a reservoirprovided in the lower end cap between the coalescing andprefilter-precoalescer subassemblies, and the air, with much oil alreadyremoved, will then pass through the coalescing layer, where additionaloil will collect in another reservoir. By providing an air flow barrieraround the lower periphery of the coalescing layer, the capacity of thefirst reservoir is increased and the oil has been found to remove itselffrom the demister by flowing back through the prefilter subassemblyagainst the flow of the air.

Accordingly, one of the objects of the present invention is to providean improved Coalescing Demister in which the resulting air flow will beof higher cleansed quality.

Another object of the present invention is to provide a CoalescingDemister which has a glass fiber prefilter-precoalescer subassemblywhich is capable of performing a prefiltering function.

Another object of the present invention is to provide a CoalescingDemister which has a coalescing subassembly that is supported by aninner and an outer rigid support tube.

Another object of the present invention is to provide a unitary glassfiber coalescing subassembly which is supported by an inner and outerrigid support tube having, at the bottom of the subassembly, an air flowbarrier which eliminates the problem of flooding both the coalescingfilter and the drain layers, and prevents re-entrainment of oil into thecleansed air stream.

A further object of the present invention is to provide a coalescingsubassembly which, by virtue of the vacuum forming of a coalescingmolded glass micro-fiber layer, will provide an interlocking compositeconstruction of the inner and outer perforated retainers.

A further object of the present invention is to provide a coalescingsubassembly of the above nature, with a synthetic fiber lateralchanneling layer to prevent the problem of channeling between theportions of the coalescing subassembly, and provide additional strengthto the demister itself, preventing damage to it due to flow or pressurefluctuations.

A further object of the present invention is to provide a CoalescingDemister having glass fiber drain and synthetic anti-migration layers asa final cleansing barrier, after the air flow has passed through thecoalescing subassembly, which will produce a cleaner out-going air flow.

A still further object of the present invention is to provide aCoalescing Demister which is resistant to rupture due to back pressureand vibration in the filter environment.

A still further object of the present invention is to provide aCoalescing Demister of the foregoing nature which may be easilyinstalled as an original, or replacement filter having either an"Out-to-In" or "In-to-Out" air flow.

A still further object of the present invention is to provide aCoalescing Demister of the foregoing nature which is relatively easy andinexpensive to manufacture.

Further objects and advantages of our invention will be apparent fromthe following description and appended claims, reference being made tothe accompanying drawings forming a part of the specification, whereinlike reference characters designate corresponding parts in the severalviews.

FIG. 1 is an exploded view of the first step in the manufacture of anout-to-in Coalescing Demister embodying the present invention.

FIG. 2 is a diagrammatic view of the process used to manufacture thecoalescing subassembly for the out-to-in Coalescing Demister.

FIG. 3 is a sectional view of the coalescing portion of the coalescingsubassembly of the out-to-in Coalescing Demister, taken in the directionof arrows on the section line 3--3 of FIG. 2.

FIG. 4 is an exploded view of the assembly sequence for the out-to-inCoalescing Demister.

FIG. 5 is a sectional view of the prefilter-precoalescer subassembly inthe direction of the arrows on the section line 5--5 of FIG. 4.

FIG. 6 is a perspective view of our out-to-in Coalescing Demisterembodying the present invention, as assembled.

FIG. 7 is a sectional view of an out-to-in Coalescing Demister embodyingthe present invention taken in the direction of the arrows on thesection line 7--7 of FIG. 6 showing the outer prefilter-precoalescersubassembly and the inner coalescing subassembly, and including lateralchanneling, glass fiber drain, and anti-migration layers, with an airflow barrier installed about the lower periphery of the coalescingsubassembly.

FIG. 8 is a fragmentary sectional view of an out-to-in CoalescingDemister embodying the present invention, showing the air flow barrierwith the physical air gap and liquid reservoir taken in the direction ofthe arrows on the section line 8--8 of FIG. 6.

FIG. 9 is an exploded view of the first step in the manufacture of anin-to-out Coalescing Demister according to the method of the presentinvention.

FIG. 10 is a diagrammatic view of the process used to manufacture thecoalescing subassembly portion of an in-to-out Coalescing Demisterembodying the present invention.

FIG. 11 is a sectional view of the coalescing portion of the coalescingsubassembly of the in-to-out Coalescing Demister Filter taken in thedirection of the arrows on the section line 11--11 of FIG. 10.

FIG. 12 is an exploded view of the assembly sequence for the in-to-outCoalescing Demister embodying the present invention.

FIG. 13 is a sectional view of the prefilter-precoalescer subassembly ofthe in-to-out Coalescing Demister taken in the direction of the arrowson the section line 13--13 of FIG. 12.

FIG. 14 is a perspective view of an in-to-out Coalescing Demisterembodying the present invention, as assembled.

FIG. 15 is a sectional view of our in-to-out Coalescing Demister takenin the direction of the arrows on the section line 15--15 of FIG. 14showing the prefilter-precoalescer subassembly, the outer coalescingsubassembly and the lateral channeling, glass fiber drain, andanti-migration layers.

FIG. 16 is a fragmentary sectional view of an in-to-out CoalescingDemister embodying our invention showing the air flow barrier with thephysical air gap and liquid reservoir taken in the direction of thearrows on the section line 15--15 of FIG. 14.

It is to be understood that the invention is not limited in itsapplication to the details of construction and arrangement of partsillustrated in the accompanying drawings, since the invention is capableof other embodiments and of being practiced or carried out in variousways within the scope of the claims. Also, it is to be understood thatthe phraseology and terminology employed herein is for the purpose ofdescription and not of limitation.

As is clear from the above description of the drawings, our CoalescingDemister can be made in two forms. The first form is referred to as anout-to-in version, and is so named because of the direction of the airflow, from the outside of the filter assembly to the inside. In thisembodiment of our invention, the air, on its travel through the filter,will come in contact with a prefilter-precoalescer subassembly held inplace by suitable restraining means, a physical air gap, an air flowbarrier, and a coalescing subassembly held in place by suitable means.

The second modification of our invention is an in-to-out demister,wherein the air flows from the interior of the demister to the outsidethereof.

The filter layers which the air will encounter on its travel through thein-to-out demister are the same as those just described for theout-to-in demister.

It should be understood that both types of demisters work equally well,and that which type demister to use depends solely on the designrequirements of the user.

For ease of understanding, the out-to-in Coalescing Demister and themethod used in its manufacture will be described first, with thedescription of the in-to-out demister to follow. In these descriptions,some of the material from our earlier co-pending application, Ser. No.593,831, filed July 7, 1975, for "Composite Coalescing Filter Tube andMethod of Manufacture Thereof" is repeated for clarity and ease ofexplanation. To ensure a complete understanding of the presentinvention, the entire contents of said earlier application, Ser. No.593,831, is specifically incorporated by reference into the presentapplication.

Referring to FIG. 1, the manufacture of the out-to-in CoalescingDemister begins with the manufacture of the coalescing portion of thecoalescing subassembly, generally designated by the numeral 54. This inturn begins with the preparation of the glass fiber slurry in the mixingtank 25.

The slurry used in forming the coalescing layer 20 of the coalescingportion of the coalescing subassembly 54 may be the same as used in thefilter tube of our earlier application, and the slurry of glass fibers,water and binder is prepared by first mixing bundles of commercial glassfibers in water with a high speed mixer 26 for about one-half (1/2)hour, so that the glass fibers will be of lengths of approximately onesixty-fourth to one-half inch in length.

The amount of glass fibers put into the water is sufficient when itforms a mixture of approximately 0.6% by weight of fibers in the water.For example, two (2) pounds of glass fibers in forty (40) gallons ofwater would provide such a mixture.

It should be understood that the amount of glass fiber added to thewater and emulsion binder mixture is not as important as the diameter ofthe glass fibers, since the percentage of glass fibers in the wateremulsion mixture can be as low as 0.1% or as high as 2.0% by weightwithout affecting the pore size of the coalescing layer 20. It is thepore size that determines the physical characteristics of the coalescinglayer, and this is controlled by adjusting the mix of the diameters ofglass fibers that are added to the water and emulsion mixture.

For example, a maximum pore size of 12 microns absolute can be obtainedby adding equal parts of glass fibers of eight (8) microns and two (2)microns in diameter. This will make a twelve (12) mircon absolutecoalescing layer, which means nothing bigger than twelve (12) micronswill be able to pass through the coalescer while in a liquid form.

The most widely accepted pore size range for the coalescing layer,however, is between eight (8) to four (4) microns absolute, and thechoice of the desired size will depend on the viscosity, quantity andsurface tension of the liquid you are coalescing.

The desired pore size in the coalescing filter can be obtained with anumber of glass fiber mixture portions and varies greatly with the typeand percentage by weight relative to the glass fibers of binder that isused.

For example, using a standard binder, at two (2) percent by weight inthe relation to the glass fibers, the following portions of glass fiberswould be needed for the indicated pore size of the filter layer, viz.

Four (4) microns absolute

16% of 0.5 micron diameter fibers

62% of 1.0 micron diameter fibers

22% of 2.0 micron diameter fibers

Six (6) microns absolute

60% of 1.0 micron diameter fibers

40% of 2.0 micron diameter fibers

Eight (8) microns absolute

22% of 1.0 micron diameter fibers

78% of 2.0 micron diameter fibers

Therefore, it is obvious that the slurry mixture will vary greatlydepending on the desired pore size of the filter to be formed.

Since several works are available in the art which give information onwhat mix of glass fiber diameters in a slurry will result in which poresizes, it is not believed necessary to discuss this matter at anygreater length in the present application. However, for the purpose ofdisclosure, we wish to note that the article entitled "Aerosolfilters-Pore size distribution in fibrous filters" by H. W. Piekarr andL. A. Clarenburg, published by the Chemical Laboratory of the NationalDefense Research Organization T.N.O., Rijswijk Z. H., The Netherlands,is particularly helpful in understanding how glass-fiber filters such asthe present one operate, and is incorporated herein by reference.

At this point, it should be understood that the present invention is notlimited to the use of glass fibers, but can be used with any othersuitable filter material.

Once the slurry is prepared, it is diluted to about 0.15% to 0.20% ofglass fibers by weight to the water before being added to the formingtank 28.

The reason for preparing the slurry in the concentrated form, and thendiluting it before placement into the forming tank 28, is that thepreparation of a concentrated slurry is more efficient, since the sizeof the mixing tank can be smaller. However, the use of such aconcentrated slurry in the forming tank 28 would be impractical becausethe forming time for any given thickness of coalescing layer would bevery short, and the outside diameter of the formed layer would be veryhard to control. Since the outside diameter of the formed layer is veryimportant for the reasons to be described, the slurry is diluted, asmentioned above, before being placed in the forming tank. Some of theslurry, for reasons to be described below, is diluted to approximately0.30% glass fibers by weight, and placed in the adder tank 30.

A synthetic cloth 21, is now securely fastened by either sewing, taping,sealing, etc., around a perforated retainer 22. This synthetic clothwill act as a lateral channeling layer 21 within the coalescingsubassembly. This assembly is now placed on a forming fixture 31 havinga rigid perforated tube 33, and secured by an end cap 34.

A vacuum from the vacuum source 35 of seven (7) to twenty-five (25)inches of Hg can be applied. Applicant has found that a vacuum ofapproximately eighteen (18 inches of Hg is most satisfactory for formingthe coalescing layer 20 when a slurry consisting of glass fiber with apreferred pore size of six (6) microns absolute is used.

The vacuum actually used will depend upon two factors, the desired speedat which the coalescing layer is to be formed, and the size of shape ofthe forming tank 28. While the amount of vacuum obtainable will dependon the kind of equipment used, it is important that not too low a vacuumbe applied, since the smaller glass fibers will not have an opportunityto migrate to the inside of the coalescing layer. The higher the vacuumthat is applied, the faster the slurry will be pulled toward the formingfixture, with the smallest fibers moving the fastest.

Further, the size of the forming tank will also have an effect on theacceptable thickness of the slurry, since a small tank would require athick slurry, while a large tank would require a thinner slurry, sincethere is a longer forming time available and, therefore, more effectivecontrol during the coalescing layer forming process.

The forming fixture 31 is left in the forming tank 28 until the vacuumgauge 38 shows a predetermined amount of resistance (vacuum) to the flowof the glass fibers has been reached. It should be understood that othermethods of regulation of the time the fixture is left in the slurry canbe used, such as a straight time controlled interval, etc., but we havefound the resistance to flow method to be one which gives a verysatisfactory control of the outer diameter of the coalescing filter,which is necessary for reasons to be explained.

The forming fixture 31 is then removed from the forming tank 28 with thevacuum still on. After a short drying time with the vacuum still on, theouter perforated retainer 36, is placed carefully over the coalescinglayer 20.

It is important that the rigid outer retainer 36, if one is used, be ofthe type which is of unitary one piece construction before installation,so it will have the necessary strength. It cannot be one, for example,which is placed around the coalescer layer 20, and then is welded orclipped together.

Once the outer perforated retainer 36, is in place, the vacuum isstopped, the end cap 34 is removed, and the assembly consisting of theinner retainer 22, lateral channeling layer 21, coalescing layer 20 andouter retainer 36 is removed. This is the coalescing portion 54 of thecoalescing subassembly.

Once the forming fixture 31 has been removed, the slurry in the formingtank 28 can be replaced as necessary from the adder tank 30 by thecontrol valve 40, and the processing of other coalescing filters cancontinue.

The adder tank 30 is not necessary, but is preferably provided, as it isdesirable to keep the slurry in the forming tank 28, at a constantconcentration to eliminate as many variables as possible which mayaffect the uniformity and quality of the coalescing layer.

As previously mentioned, the slurry in the adder tank 30 has alreadybeen diluted to about one-half (1/2) of the concentration of the glassfiber slurry in the mixing tank 25 for the convenience of the prefilterand filter processing operations.

The coalescing portion of the coalescing subassembly will now be dried.The exact drying time will vary widely depending on the temperature atwhich the drying operation takes place, as well as the velocity of thedrying air. However, a minimum drying temperature of about 200° F. isnecessary to turn the emulsion binder in the glass fiber slurry to asolid. It is obvious that this operation can be done many ways. Thus, noparticular way of doing this is set out, and the entire operation isgenerally designated by the numeral 44.

Once the coalescing portion of the coalescing subassembly is air dried,it will be placed into an epoxy dip 45, which is at room temperature,until it becomes saturated. It is then removed from the epoxy dip andonce again air dried, this time to remove the solvents from the epoxy.This second air drying operation is done at a slightly lowertemperature, about 180° F., than the first air drying operation, due tothe flammability of the solvents in the epoxy.

The coalescing portion 54 is then oven cured 46 to provide the necessarystrength to permanently hold it together. Note that any suitable epoxycan be used, with the choice depending on the particular use for whichthe filter is intended.

However, since commercial epoxides are made for paints, it will benecessary to thin them before being used. A suggested thinning rangebeing between ten (10%) and fifty (50%) by weight in relation to theweight of the coalescing layer. The standard practice in the art is touse a thinning ratio of twenty (20).

The coalescing portion 54 is placed into the curing oven 46 forapproximately 1/2 hour at a temperature of 280° F. Note that thetemperature used in the curing oven 46 will depend on the type of epoxybinder used and the types of retaining tubes. For certain applications,it is possible to use plastic inner and outer retaining tubes tosurround the coalescing layer 20, and in this case the temperature inthe curing oven would be approximately 200° F. rather than the 280° F.which is used when metal retaining tubes are used.

Once the curing 46 is finished, the processing of the coalescing portion54 is completed, and the assembling of the remaining parts of theCoalescing Demister can begin.

A sectional view of the completed coalescing portion 54 of thecoalescing subassembly of the out-to-in Coalescing Demister is shown inFIG. 3, with its four (4) component parts being an inner perforatedretainer 22, a synthetic fiber lateral channeling layer 21, a coalescinglayer of glass micro-fibers 20, and the outer perforated retainer 36.

If an in-to-out Coalescing Demister were being manufactured, thecoalescing portion 54 shown in FIG. 11 would be manufactured in exactlythe manner just described. The only possible difference would be thesize of the components 22, 21, 20 and 36. These in all likelihood wouldbe larger for the in-to-out demister, but not necessarily so. The sizewould depend on the application.

The next step in the manufacture of the demister is to install the airflow barrier on the outer perforated retainer 36 at a lower end 37thereof. This air flow barrier 41 can be made of any nonporous materialwhich would prohibit passage of air.

It should be noted that the air flow barrier need not be attached to theouter perforated retainer, but may take the form of a separate annularband spaced a short distance from the lower edge 37 and held in place bythe potting compound. In this manner the filter area behind the airbarrier remains active. Whether mounted on the outer retainer, or merelymounted about the lower periphery thereof, we have found that the airflow barrier will prohibit the flooding of the coalescing and drainlayers of the filter as previously discussed, and will facilitate thegravitational removal of the oil without re-entrainment into thecleansed air stream. The use of this air flow barrier is a novelimprovement over the prior art, and solves a long standing problemtherein.

Referring to FIG. 4, the glass fiber drain layer 45 is placed inside theinner retainer 22 in a manner to ensure that there is intimate contactbetween the drain layer and the inside of the retainer 22.

Next, the anti-migration layer 46, is placed inside the glass fiberdrain layer 45 and expanded into place. Once the anti-migration layer 46is secured, which may be done by any suitable means, a screen-typeretainer 47 is placed into position to support the two filter layers.The screen-type retainer 47 may be made of any suitable material whichwill adequately support the layers 45 and 46 under the particularoperating conditions for which the demister is intended.

We have found that the most suitable material to use for theanti-migration layer 46 is a synthetic cloth, although it is to beunderstood that others can also be used.

Referring to FIG. 12, if an in-to-out Coalescing Demister were beingmanufactured, the steps of providing the drain layer 45, theanti-migration layer 46 and the screen-type retainer 47 would besubstantially similar to those just described, only the sizes of thecomponents, 45, 46 and 47 would be different.

Because of the reversal of the air flow, the anti-migration layer 46would not come after the drain layer 45 which would now be in intimatecontact with the outer retainer tube 35. The screen-type retainer 47would then be placed around the anti-migration layer 46.

Again referring to FIG. 4, with the layers 45 and 46, and retainer 47 inplace inside of the retainer 22, the coalescing assembly is complete.

The prefilter-precoalescer subassemblies of FIG. 5 and FIG. 13 is nowassembled. A first prefilter retainer 48 is wrapped with a double layerof glass fiber cloth 49. Once the double wrap of glass fiber cloth 49 isin place, the second prefilter screen-type retainer is provided bywrapping a layer of suitable material around the fiber cloth 49 andfastening it securely by any suitable means such as soldering, the useof metal clips, etc.

With the coalescing and prefilter-precoalescer subassemblies nowcompleted, the out-to-in Coalescing Demister of FIG. 4 may now beassembled. A lower end cap 51, whose shape depends on the particularapplication in which the demister is to be used, is filled with pottingcompound.

Since several works are available in the art which give information onthe type of potting compound which can be placed in the end cap, it isbelieved unnecessary to discuss this matter at length in the presentapplication. However, for purpose of clarity, we wish it known that anysuitable epoxy, vinyl, phenolic, polyurethane or silicone pottingcompound, among others, can be used.

Once the end cap 51 has the nonporous adhesive sealer in place, theprefilter-precoalescer subassembly 55 and the coalescing subassembly 54and the air barrier 41 are potted therein, making certain the edgeregions of both subassemblies are completely sealed. The upper end cap52 is similarly potted in place, with the finished product shown in FIG.6.

In regard to the assembly of the in-to-out Coalescing Demister, severalitems where the manufacturing process are different have already beendiscussed in regard to the coalescing subassembly, the air flow barrierand the prefilter-precoalescer subassembly. Since aside from thesedifferences, the processes involved are substantially similar themanufacture of the in-to-out demister may briefly be set forth byreferring to FIGS. 9-12.

Referring to FIG. 9, it is apparent that the manufacture of the"In-to-Out" Coalescing Demister, like the "Out-to-In" CoalescingDemister, begins with the manufacture of the coalescing subassembly andthe wrapping of the anti-channeling layer 21 around the enlarged innerretainer 22. The coalescing layer 20 is then formed as before in theslurry tank 28 with the use of the forming fixture 31, end cap 34 andvacuum source 35, taking into account the same factors as to slurry mix,amount of vacuum, replacement of used slurry, etc., as was done in themanufacture of the out-to-in demister just described.

The forming fixture 31 is now removed from the slurry mix with thecoalescing layer 20 now formed over the synthetic cloth anti-channelinglayer 21, and the outer retainer 36 is installed, thus completing thecoalescing portion 54 of the coalescing subassembly.

Since the air flow is from out-to-in in this modification of ourinvention, the outer retainer 36 may not be required for someapplications of our invention, and in this case the inner retainer 22with the coalescing layer 20 is simply removed from the forming fixture31 after the vacuum is shut off.

The coalescing portion 54 is now air dried 44, epoxy dipped 45, againair dried 44 and cured 46.

The drain layer 45 is now wrapped around the outside of the outerretainer 36 without fastening, and this in turn is wrapped with ananti-migration layer 46. The layers 45 and 46 are held in place by ascreen-type retainer 47 which is fastened securely about the layer 46 byany suitable means.

The air flow barrier 41 is now installed on the inside of the innerperforated retainer 22 at the bottom thereof in the manner previouslydescribed.

The prefilter-precoalescer subassembly 55 is now assembled, also aspreviously described, but is much smaller in size to fit inside thecoalescer subassembly and still provide the physical air gap 53.

The coalescing and prefilter-precoalescer subassemblies are now pottedto lower 51 and upper 53 end caps and the demister is complete.

Although it is to be understood that our invention is not limited to usein air compressors, but is suitable for use in any application whereinoil entrained in an air or gas stream must be removed, such as oilquenching operations or industrial smoke stacks, for ease ofillustration we will now describe the actual operation of our filter inconnection with its use in an air compressor.

In actual operation, when the air with entrained oil enters theout-to-in demister through the prefilter-precoalescer subassembly 55 andexits through the coalescing subassembly, if the glass fiber cloth 49 ischosen correctly, so that the pores therein are large enough to allowthe air to pass through the filter with the oil entrained therein beingcoalesced properly, the coalesced oil will now run down the inside ofthe prefilter-precoalescer because of the nonwicking properties of thematerial, and collect in the reservoir 56. To prevent the oil in thereservoir from flooding the coalescing subassembly an air flow barrier41 is provided aroud the lower periphery of the outer retainer 36. Whilethis oil could be scavenged from the reservoir 56 and added feature ofour invention is that by the proper choosing of the prefilter media,once the oil reservoir is filled to above the rim of the lower end cap51 the oil, by the force of gravity, will drain back through theprefilter subassembly 55 and thus drain itself.

The air, with a substantial portion of the entrained oil alreadyremoved, then passes through the coalescing portion 54 and through thedrain and anti-migration layers 45 and 46, where further oil iscoalesced and runs down the inside of the subassembly to collect in thedish shape portion 57 of the end cap, with the air then exiting throughthe suitable opening in the upper end cap 52. The operation of thein-to-out demister would be substantially similar, except that the oil,instead of collecting in the dish-shaped portion 57, will merely exitthrough the opening in the end cap and return to the oil sump.

Thus, by abandoning the previous theory of operation of CoalescingDemisters, in which it was felt that the air stream with the oilentrained therein must first pass through as tight a coalescing layer aspossible before being passed through a drain layer, and providing ademister having a prefilter-precoalescing stage through which the air ispassed before passing through a coalescing filter layer, the objects ofthe present invention listed above and numerous additional advantagesare attained.

We claim:
 1. A Coalescing Demister including a prefilter-precoalescersubassembly including at least one layer of a suitable filter materialwhose properties include being non-wicking and having a pore sizeproperly chosen to coalesce a major portion of the oil entrained in anair stream and still allow gravitational backflow of oil, and acoalescing subassembly having a proper pore size to coalesce virtuallyall of the oil particles remaining in said air stream after passingthrough said prefilter-precoalescer subassembly and being concentricallylocated with respect to said prefilter-precoalescer subassembly andhaving a physical air gap there between, and having said subassembliespotted between upper and lower end caps to seal the edge regions of saidsubassemblies to prevent air leakage and to maintain said air gap andincluding an air flow barrier mounted in an appropriate position on thelower periphery of said coalescing subassembly and having at least onereservoir formed between said air flow barrier and saidprefilter-precoalescer subassembly to collect oil and aid saidgravitational backflow of oil.
 2. The device defined in claim 1, andincluding an air flow barrier mounted about the lower periphery of saidcoalescing subassembly.
 3. An out-to-in Coalescing Demister including aninner coalescing subassembly including at least one layer of a suitablefilter material whose properties include being non-wicking and having apore size properly chosen to coalesce a major portion of the oilentrained in an air stream and still allow gravitational backflow ofoil, and an outer coalescing subassembly having a proper pore size tocoalesce virtually all of the oil particles remaining in said air streamafter passing through said prefilter-precoalescer subassembly and beingconcentrically located with respect to said coalescing subassembly withan air gap there between, a lower end cap potted to the lower edgeregions of both of said subassemblies and an upper end cap potted to theupper edge regions of said subassemblies to prevent air leakage and tomaintain said subassemblies in a concentric relationship, and includingan air flow barrier mounted in an appropriate position about the outerlower periphery of said coalescing subassembly and having at least onereservoir formed between said air flow barrier and saidprefilter-precoalescer subassembly to collect oil and to aid saidgravitational backflow of oil.
 4. The device defined in claim 3, whereinsaid prefilter-precoalescer subassembly includes a first prefilterretainer, at least one layer of suitable glass filter fiber clothwrapped around said retainer and a second prefilter retainer securedover said glass filter fiber cloth.
 5. The device defined in claim 3,wherein said coalescing subassembly includes an inner perforatedretainer, a layer of glass fiber cloth suitable for performing thefunction of an anti-channeling filter layer in intimate contact withsaid inner retainer, a layer of glass fibers formed over saidanti-channeling layer, and an outer perforated retainer placed over saidglass fiber coalescing layer.
 6. The device defined in claim 5, withsaid outer perforated retainer being omitted.
 7. The device defined inclaim 5, wherein said prefilter-precoalescer subassembly includes afirst prefilter retainer at least one layer of suitable glass filterfiber cloth wrapped around said retainer and a second prefilter retainersecured over said glass filter fiber cloth.
 8. The device defined inclaim 7, and including at least one layer of a glass fiber clothsuitable for performing the function of a drain layer in initimatecontact with said inner perforated retainer, at least one layer of asynthetic cloth suitable for performing the function of ananti-migration layer in intimate contact with said drain layer, and ascreen-type retainer in operative engagement with said anti-migrationlayer.
 9. The device defined in claim 8, wherein said air flow barrieris made of a metallic material.
 10. The device defined in claim 8, withsaid air flow barrier mounted on the lower periphery of said outerperforated retainer.
 11. The device defined in claim 10, wherein saidair flow barrier is made of plastic tube.
 12. The device defined inclaim 10, with said outer perforated retainer being omitted, and saidflow barrier being mounted about the lower periphery of said glass fiberlayer.
 13. An in-to-out Coalescing Demister including an innerprefilter-precoalescer subassembly including at least one layer of asuitable filter material whose properties include being non-wicking andhaving a pore size properly chosen to coalesce a major portion of theoil entrained in an air stream and still allow gravitational backflow ofoil, and an inner coalescing subassembly having a proper core size tocoalesce virtually all of the oil particles remaining in said air streamafter passing through said prefilter-precoalescer subassembly and beingconcentrically located with respect to said prefilter-precoalescersubassembly with a physical air gap there between, a lower end cappotted to the lower edges of said subassemblies and an upper end cappotted to the upper edges of said subassemblies to prevent air leakageand to maintain the concentric relationship between said subassemblies,and including an air flow barrier mounted in an appropriate positionabout the inner lower periphery of said coalescing subassembly andhaving at least one reservoir formed between said air flow barrier andsaid prefilter-precoalescer subassembly to collect oil and to aid saidgravitational backflow of oil.
 14. The device defined in claim 13,wherein said coalescing subassembly includes an inner perforatedretainer, at least one layer of a glass fiber cloth suitable forperforming an anti-channeling function secured in initimate contact withsaid inner perforated retainer, a coalescing layer of suitable glassfibers formed over said anti-migration layer, and an outer perforatedretainer mounted over said coalescing layer in intimate contacttherewith.
 15. The device defined in claim 13, wherein saidprefilter-precoalescer subassembly includes a first prefilter retainer,at least one layer of glass fiber cloth suitable for performing aprefilter-precoalescer function in intimate contact with said firstretainer and a second prefilter retainer secured over said glass fibercloth.
 16. The device defined in claim 15, wherein said coalescingsubassembly includes an inner perforated retainer, at least one layer ofa synthetic cloth suitable for performing an anti-channeling functionsecured in intimate contact with said inner perforated retainer, acoalescing layer of suitable glass fibers formed over saidanti-migration layer, and an outer perforated retainer mounted over saidcoalescing layer in intimate contact therewith.
 17. The device definedin claim 16, with said outer perforated retainer being omitted.
 18. Thedevice defined in claim 16, and including at least one layer of a glassfilter fiber cloth suitable for performing as a drain layer in intimatecontact with said outer perforated retainer at least one layer of asynthetic cloth suitable for an anti-migration layer in intimate contactwith said drain layer and a screen-type retainer secured over said drainlayer.
 19. The device defined in claim 18, wherein said air flow barrieris mounted on the lower periphery of the inside of said inner perforatedretainer.
 20. The device defined in claim 19, wherein said flow barrieris made of plastic tube.
 21. The device defined in claim 19, whereinsaid air flow barrier is made of a metallic material.
 22. The devicedefined in claim 19, with said inner perforated retainer being omitted,and said air flow barrier being mounted on the lower periphery of saidglass fiber layer.